Digital data transmission has been carried out with various forms of modulation. The particular modulation format which is used in a system impacts the spectrum occupancy of the transmitted signal and the equipment complexity for both the modulator and demodulator. Ordinary bi-phase phase shift keying (BPSK) modulation is noted for its wide spectrum occupancy because of its instantaneous phase transitions. The phase transitions cannot be removed by filtering if the signal is subsequently to be processed by non-linear transmitter stages. However, a BPSK waveform can be demodulated with a relatively simple correlator circuit. Staggered quadraphase phase shift keying (SQPSK) modulation has been utilized to alleviate this problem. The use of SQPSK results in a more complex transversal correlator since the transversal correlator must identify whether the pseudo noise (PN) modulation data stream is to be found in the in-phase (I) or quadrature-phase (Q) channel for each of the two independent data streams which are transmitted in the I or Q channels. For SQPSK modulation four parallel traversal correlators are required to properly demodulate an incoming signal with arbitrary phase.
The principle problem with BPSK modulation is that it contains no energy at other than 0 degree and 180 degree phase angles. Therefore, there is no phase rotation to permit a follow-up filter to provide a smooth phase rotation. Without a phase rotation, there must be an amplitude collapse in order to change the phase state. This amplitude collapse causes spectrum spreading in class C transmitting amplifiers or limiting devices. One modification to standard BPSK involves the transitory injection of quadrature energy during a bit transition so that a smooth phase rotation can be achieved. This injection can be produced by the technique illustrated in U.S. Pat. No. 3,173,201 or through the use of the SQPSK technique.
SQPSK modulation has the virtue of greater utilization of the available bandwidth by employing the unused quadrature channel. As a disadvantage, however, the threshold for deciding whether a transmitted bit is a one or a zero, is halved by requiring the system to differentiate 90 degree phase increments rather than 180 degree phase increments for data modulation systems. Further, there is the need for four transversal correlators in a spread spectrum application.
Therefore there exists a need for a method and apparatus for reducing the spectral occupancy of a MSK related signal while using a relatively simple transversal correlator to demodulate the signal.